Maybe you want to start running in the evenings, swap your afternoon cookie for an apple, or be more reliable about a particular business practice. Whatever habit you're aiming for, everyone knows it's going to take a awhile.

Changing habits isn't easy (though it is possible, my colleague Jeff Haden has laid out how in detail), but the questions remains just how long should you expect it to take for the new routine to start feeling natural. Underestimate this period, and it's easy to feel discouraged when it's taking longer than expected for the new habit to stick.

Overestimate the period too greatly and you might not even bother giving it a try.

21 Days?

There is one number floating out there in the popular imagination you may have heard -- 21 days. As entrepreneur James Clear explains on the iDoneThis blog, this number actually originated with a plastic surgeon who found it took three weeks for patients to get used to their changed appearance. From this fact he extrapolated that it must take this long for habits to form. Various self-help gurus have been repeating 21 days ever since.

The only trouble with this number is it appears to be wrong. Researchers at University College London actually did a more detailed study of how long it took volunteers to establish a habit of their choosing, from drinking more water with lunch to undertaking a fitness program. Rather than three weeks, the average length of time before study participants felt the behavior had become natural was... 66 days.

For some people it took as long as eight months.

Going the Distance

Two months or more is obviously a heck of a lot longer than three weeks, so how can you keep up your quest to form a new habit despite this considerably longer slog? First, you're now prepared. If you were expecting things to sort themselves out in under a month, it's no wonder you'd get frustrated when it still feels like a struggle after a month.

But there are also several tips and tricks that can help you go the distance. First, research shows that motivation often flags about halfway to a goal. In the beginning we're fired up to start; at the end our destination is in sight. It's the middle of the journey that's dispiriting. So prepare for that lull in motivation and remember to look back at how far you've come and ahead at how much you are set to gain when your dedication flags.

Another technique to get you down the long road to a new goal is simple self-bribery. Say you want to drink that healthful bottle of water at lunch. Promise yourself that while you do it, you'll watch a favorite funny video or check the sports scores. The idea is to pair the less than pleasant new habit to some small indulgence to help you get over any bumps in the road.

As for Clear, his takeaway is simple: embrace the longer timeline and focus on the process, not the destination. As he's written elsewhere, you're more likely to achieve success by focusing on consistently practicing small daily actions than your final end goal.